Retina-dependent activation by apomorphine of metabolic activity in the superficial layer of the superior colliculus

Quantification of indirect pathway inhibition by the adenosine A2a antagonist SYN115 in Parkinson disease

Adenosine A(2a) receptor antagonists reduce symptom severity in Parkinson disease (PD) and animal models. Rodent studies support the hypothesis that A(2a) antagonists produce this benefit by reducing the inhibitory output of the basal ganglia indirect pathway. One way to test this hypothesis in humans is to quantify regional pharmacodynamic responses with cerebral blood flow (CBF) imaging. That approach has also been proposed as a tool to accelerate pharmaceutical dose finding, but has not yet been applied in humans to drugs in development. We successfully addressed both these aims with a perfusion magnetic resonance imaging (MRI) study of the novel adenosine A(2a) antagonist SYN115. During a randomized, double-blind, placebo-controlled, crossover study in 21 PD patients on levodopa but no agonists, we acquired pulsed arterial spin labeling MRI at the end of each treatment period. SYN115 produced a highly significant decrease in thalamic CBF, consistent with reduced pallidothalamic inhibition via the indirect pathway. Similar decreases occurred in cortical regions whose activity decreases with increased alertness and externally focused attention, consistent with decreased self-reported sleepiness on SYN115. Remarkably, we also derived quantitative pharmacodynamic parameters from the CBF responses to SYN115. These results suggested that the doses tested were on the low end of the effective dose range, consistent with clinical data reported separately. We conclude that (1) SYN115 enters the brain and exerts dose-dependent regional effects, (2) the most prominent of these effects is consistent with deactivation of the indirect pathway as predicted by preclinical studies; and (3) perfusion MRI can provide rapid, quantitative, clinically relevant dose-finding information for pharmaceutical development.

Astrocyte activation in vivo during graded photic stimulation

Astrocytes have important roles in control of extracellular environment, de novo synthesis of neurotransmitters, and regulation of neurotransmission and blood flow. All of these functions require energy, suggesting that astrocytic metabolism should rise and fall with changes in neuronal activity and that brain imaging can be used to visualize and quantify astrocytic activation in vivo. A unilateral photic stimulation paradigm was used to test the hypothesis that graded sensory stimuli cause progressive increases in the uptake coefficient of [2-(14)C]acetate, a substrate preferentially oxidized by astrocytes. The acetate uptake coefficient fell in deafferented visual structures and it rose in intact tissue during photic stimulation of conscious rats; the increase was highest in structures with monosynaptic input from the eye and was much smaller in magnitude than the change in glucose utilization (CMR(glc)) by all cells. The acetate uptake coefficient was not proportional to stimulus rate and did not correlate with CMR(glc) in resting or activated structures. Simulation studies support the conclusions that acetate uptake coefficients represent mainly metabolism and respond to changes in metabolism rate, with a lower response at high rates. A model portraying regulation of acetate oxidation illustrates complex relationships among functional activation, cation levels, and astrocytic metabolism.

Altered thalamic response to levodopa in Parkinson's patients with dopa-induced dyskinesias

Parkinson's disease (PD) is a progressive neurologic condition characterized by tremor, slowness, stiffness, and unstable posture. Degeneration of dopamine-producing neurons in the substantia nigra causes PD. Treatment with levodopa, a precursor of dopamine, initially ameliorates the clinical manifestations of PD. However, chronic levodopa treatment can produce severe involuntary movements (so-called dopa-induced dyskinesias or DID), limiting treatment. Pallidotomy, placement of a surgical lesion in the internal segment of the globus pallidus, reduces DID. Because this result is inconsistent with current theories of both basal ganglia function and DID, it prompted us to investigate the brain's response to levodopa. We measured regional cerebral blood flow response to levodopa with positron-emission tomography in 6 PD patients with DID, 10 chronically treated PD patients without DID, 17 dopa-naïve PD patients, and 11 normals. The dose of levodopa was chosen to produce clinical benefit without inducing DID. This strategy allowed us to examine the brain response to levodopa across groups without the confounding effect of differences in motor behavior. We found that the DID group had a significantly greater response in ventrolateral thalamus than the other groups. This was associated with decreased activity in primary motor cortex. These findings are consistent with increased inhibitory output from the internal segment of the globus pallidus to thalamus after levodopa administration. They provide a physiological explanation for the clinical efficacy of pallidotomy and new insights into the physiology of the basal ganglia.

The reinforcing properties of nicotine are associated with a specific patterning of c-fos expression in the rat brain

Rats were trained for nicotine intravenous infusions in a self-administration paradigm. The effect of nicotine self-administration on regional brain activity was studied by mapping changes of c-fos expression. Specific nicotine effects were determine by comparing the patterning of Fos-like immunoreactivity (Fos-Ll) in nicotine self-administering rats with that in three different control groups. Controls included rats exposed to the same manipulation as nicotine self-administering rats who received intravenous saline instead of nicotine. In addition, two groups of untrained sham-operated rats exposed daily to the same operant boxes were included: one group had the same food restriction used in the operant training, the other was fed ad libitum. Nicotine self-administration, exposure to saline and food restriction increased Fos-Ll in 43, 33 and three brain regions, respectively, when compared with the control group fed ad libitum. Computer-assisted image analysis of Fos-Ll profiles performed on 16 relevant limbic and sensory structures showed that in saline-treated rats a significant (P < 0.01) increase of Fos-Ll profiles was observed in medial prefrontal cortex, lateral septum, core and ventral shell of nucleus accumbens, claustrum, amygdaloid nuclei, paraventricular thalamic nucleus and lateral geniculate nucleus. A significant (P < 0.01) further increase produced by nicotine was found in medial prefrontal cortex and ventral shell of nucleus accumbens. Interestingly, cingulate and piriform cortex, superior colliculus and medial terminal nucleus of the accessory optic tract were specifically activated by nicotine but not saline. These results show that nicotine self-administration activates sensory structures, as well as limbic structures involved in natural rewarding pathways. The results suggest the involvement of restricted terminal regions of the mesocorticolimbic dopaminergic system in the maintenance of nicotine self-administration.

Dopamine and serotonin turnover rate in the retina of rabbit, rat, goldfish, and Eugerres plumieri: light effects in goldfish and rat

The concentration of dopamine, and its metabolites 3,4-dihydroxyphenylacetic and homovanillic acids, as well as serotonin and its metabolite 5-hydroxyindoleacetic acid, were determined in the retina of two teleosts, C. auratus (goldfish) and E. plumieri (mojarra), and two mammals, R. norvegicus (rat) and O. cuniculus (rabbit). The turnover rate of these monoamines were investigated in the four species by the calculation of the ratio monoamine/metabolite as an indirect index, and in goldfish and rat by the inhibition of the synthesis with alpha-methyl-p-tyrosine or p-chlorophenylalanine, by the increase in dopamine or serotonin by the corresponding precursors, 3,4-dihydroxyphenylalanine or 5-hydroxytryptophan, and by inhibition of monoaminooxidase with pargyline. The modulation by light and dark stimulation was studied in the goldfish and the rat. Differences in the concentration and turnover rate were observed among the species. Serotonin concentration was higher in the teleosts. The administration of inhibitors of dopamine and serotonin synthesis differentially decreased the levels of the monoamines in the retina of goldfish and rat. The rate of formation of dopamine and serotonin by the corresponding precursors was much higher in the goldfish than in the rat. Pargyline administration decreased 3,4-dihydroxyphenylacetic and 5-hydroxyindoleacetic acids at different rates and time dependency in the retina of goldfish and rat. Dopamine and serotonin concentration did not exhibit high modifications by the inhibitor, suggesting the function of regulatory mechanisms or additional effect of pargyline at other sites different from monoaminooxidase.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic mapping of the effects of intravenous methamphetamine administration in freely moving rats

The 2-[14C]deoxyglucose method was used to examine the effects of acute intravenous administration of methamphetamine (0.5-2.5 mg/kg) on rates of local cerebral glucose utilization in freely-moving rats. These effects were correlated with the effects of methamphetamine on locomotor activity assessed simultaneously in the same animals. Methamphetamine administration resulted in widespread dose-dependent increases in glucose utilization within structures of the extrapyramidal motor system. Rates of glucose utilization were positively correlated with locomotor activity in the globus pallidus, substantia nigra reticulata, entopeduncular nucleus, subthalamic nucleus, and the lateral cerebellar cortex. In contrast, within the limbic system alterations in metabolic activity were smaller and more selective. Glucose utilization was increased in the nucleus accumbens at all doses tested, but alterations in glucose utilization in the ventral tegmental area, amygdala, and anterior cingulate were observed only at the highest doses of methamphetamine tested. Significant increases in rates of glucose metabolism were also found in the substantia nigra compacta and in the median and dorsal raphe nuclei. Dopamine and serotonin are depleted in these regions, as well as in the ventral tegmental area where glucose utilization was also increased, following chronic treatment with high doses of methamphetamine. These changes in glucose utilization may be indicative of disturbances in the biochemical processes involved in the neurotoxic effects of methamphetamine.

Dopamine synthesis and metabolism in rhesus monkey retina: development, aging, and the effects of monocular visual deprivation

The normal postnatal development, the influence of age, and the effects of visual deprivation on the dopamine system in the retina of rhesus monkeys were examined. The lowest level of retinal dopamine was found at birth. By 3-4 weeks of age, the dopamine concentration had more than doubled. This level remained relatively constant in the retinas of older infants and of adult monkeys up to 34 yr of age. The level of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) and the activity of tyrosine hydroxylase did not significantly change as a function of age during the postnatal life span. Monocular occlusion of newborn or infant monkeys for 1-15 months with opaque contact lenses resulted in decreases in the retinal concentrations of dopamine and DOPAC relative to the concentrations in the same animals' unoccluded eyes. Occlusion also resulted in a lower level of tyrosine hydroxylase activity in the retina. Monocular eyelid suture from birth to 15 months of age resulted in less consistent alterations of retinal dopamine and DOPAC levels; tyrosine hydroxylase activity, however, was consistently reduced by lid suture. Thus, dopamine synthesis and metabolism, and the ontogenetic increase of the retinal dopamine level of rhesus monkey are reduced by light deprivation.

Detection of visual stimuli after lesions of the superior colliculus in the rat; deficit not confined to the far periphery

The visual neglect which follows lesions of the superior colliculus in the rat is well documented, although its nature has been somewhat contentious. Recently, using large, bright stimuli, Overton et al. [Exp. Brain Res., 59 (1985) 559-569] presented evidence that collicular lesions produce a detection deficit in the far peripheral visual field (120 degrees), whilst the central field is unaffected. However, some simple tests for visual neglect by other workers suggest that a detection deficit may also exist in the central field, if the stimuli are dim (and small). This possibility was investigated psychophysically. Using a signal detection paradigm, sensitivity to small (2.5 degrees v.a.), transient (modal duration 200 ms) stimuli of varying luminance was determined at 40 degrees on either side of the vertical meridian, before and after collicular lesions or control lesions of the striate cortex. Despite extensive preoperative training, collicular animals showed a decrease in sensitivity of around 50% for all stimulus intensities postoperatively. This deficit was significantly more severe than that produced by striate lesions. Furthermore, it proved very difficult to retrain the animals to a preoperative criterion. These data suggest that collicular lesions do indeed cause a detection deficit which is not confined to the far periphery, and that this deficit may specifically affect the detection of small stimuli. The proposed detection deficit is related to recent anatomical findings in the rat.

Nicotine enhances cerebral glucose utilization in central components of the rat visual system

The effect of nicotine in visual system components of the rat brain was examined using the 2-deoxy-D-[1-14C]-glucose ([14C]DG) technique. Nicotine was administered subcutaneously (SC) at doses of 0.1, 0.3, and 1.0 mg/kg 2 min before the infusion of the radiotracer. Local cerebral glucose utilization (LCGU) was stimulated by nicotine in a dose-dependent manner in many brain regions associated with the visual system. Increases of over 100% were seen in the superior colliculus, nucleus of the optic tract, and portions of the accessory optic system (medial and dorsal terminal nuclei, and the inferior fasciculus). Statistically significant increases were also observed in the lateral geniculate nucleus, the lateral terminal nucleus and the cerebellum. The effects were blocked by pretreatment with mecamylamine and by enucleation. The findings lend support to the involvement of the nicotinic cholinergic system in the processing of visual information or visual-motor function.

Catecholamine systems of retina: a model for studying synaptic mechanisms

The retina contains three catecholamine neurotransmitters: dopamine (DA); norepinephrine (NE); and epinephrine (EPI). DA and EPI appear to be associated with separate amacrine neurons that directly participate in the visual process. NE, in contrast, appears to be associated primarily with the sympathetic nerves that innervate the blood vessels of the retina. We present a synopsis of the anatomy, physiology, biochemistry and pharmacology of these retinal neurons. We also suggest that some diseases usually associated with catecholamines of brain may have their counterpart in retina.

Cluster-and-sheet pattern of enkephalin-like immunoreactivity in the superior colliculus of the cat

The distribution of enkephalin-like immunoreactivity in the superior colliculus has been studied in the cat with the peroxidase-antiperoxidase method. Two striking patterns of immunoreactivity were observed. In the superficial layers there is a thin, dense horizontal band of immunoreactivity in the neuropil of the most dorsal tier of the superficial gray layer (sublamina 1). Because this sublayer corresponds to the zone of densest contralateral retinotectal projection, an intraocular injection of horseradish peroxidase was made in one cat to allow direct comparison of the distributions of opiate-like immunoreactivity and transported tracer in the contralateral superior colliculus. There was a detailed similarity between the two, including the presence of a gap in both at the presumptive site of the optic disc representation. The presence of enkephalin-like immunoreactivity in neural perikarya in and near sublamina 1 of the superficial gray layer, however, raised the possibility that the immunoreactive band is part of an intrinsic opiate system. Deeper in the superficial gray layer there was appreciable but weaker immunoreactivity in the neuropil and fewer immunoreactive neurons. In the intermediate gray layer and, especially medially, even deeper in the superior colliculus, enkephalin-like immunoreactivity was organized into small (100-300 micron wide) patches. In the intermediate gray layer these tended to be arranged periodically, five-seven patches being spaced at 200-600 micron intervals in caudal transverse sections. In some sections adjoining patches appeared to be fused. The patches were absent or difficult to detect in rostral sections. Caudally, they sometimes were adjacent to blood vessels penetrating the intermediate gray layer, but other times were not. Serial section reconstructions suggested that the patches observed in individual sections are part of larger arrays which have the form of anastomotic bands running in longitudinal directions somewhat oblique to the sagittal plane. It is concluded that an opiate mechanism may play a part in controlling the effects of incoming retinal information in the superficial gray layer, directly or indirectly, and that opiate peptides may also act in modulating one or more afferent or efferent systems of the deep collicular layers. Accordingly, from the functional standpoint, enkephalin-like peptides may influence both visual and sensory motor processing in the superior colliculus.

Functional consequences of unilateral lesion of the locus coeruleus: a quantitative [14C]2-deoxyglucose investigation

The functional consequences, as reflected in local rates of glucose utilization, of ablation of the locus coeruleus (the nucleus from which a major portion of the ascending noradrenergic fibres arise) have been examined in conscious rats with the quantitative autoradiographic [14C]2-deoxyglucose technique. Measurements of glucose utilization were made 72 h after histologically verified unilateral electrolytic lesions of the locus coeruleus. In the overwhelming majority of the 35 grey matter regions examined, the rate of glucose utilization was unaltered by lesions of the locus coeruleus, and in the limited number of CNS regions in which significant alterations were observed, the magnitude of the changes was invariably modest (less than 20% different from sham-operated control animals). Reductions in glucose use were observed in ipsilateral ventral (by 14%) and lateral thalamic nuclei (by 17%), and rates of glucose utilization in most regions of cerebral cortex were significantly lower (about 10%) in the ipsilateral hemisphere relative to the hemisphere contralateral to the lesion. In one region, the median raphe nucleus, glucose utilization was significantly elevated (by 19%) following lesions of the locus coeruleus. Attempts to accentuate the effects of locus coeruleus lesions by pharmacological manipulation of CNS adrenoreceptors by means of the systemic administration of phenoxybenzamine (30 mg/kg, 40 min prior to measurement of glucose use) in animals bearing unilateral locus coeruleus lesions were unsuccessful; the modest alterations in glucose utilization observed following locus coeruleus lesion alone were even less pronounced in lesioned animals receiving phenoxybenzamine. The alterations in local glucose utilization provoked by phenoxybenzamine were similar in sham-lesioned and locus coeruleus-lesioned animals. It would appear that the functional consequences, in terms of glucose utilization, are much less pronounced when a single neurotransmitter system (in the present studies, noradrenergic neurones) is lesioned than when a multiple neurotransmitter, functionally integrated pathway (such as the visual system) is disrupted.

The effects of apomorphine upon local cerebral glucose utilization in conscious rats and in rats anesthetized with chloral hydrate

The effects of the dopaminergic agonist apomorphine (1 mg . kg-1 i.v.) upon local cerebral glucose utilization in 43 anatomically discrete regions of the CNS were examined in conscious, lightly restrained rats and in rats anesthetized with chloral hydrate by means of the quantitative autoradiographic [14C]2-deoxyglucose technique. In animals anesthetized with chloral hydrate, glucose utilization was reduced throughout all regions of the CNS from the levels observed in conscious animals, although the magnitude of the reductions in glucose use displayed considerable regional heterogeneity. With chloral hydrate anesthesia, the proportionately most marked reductions in glucose use (by 40-60% from conscious levels) were noted in primary auditory nuclei, thalmaic relay nuclei, and neocortex, and the least pronounced reductions in glucose use (by 15-25% from conscious levels) were observed in limbic areas, some motor relay nuclei, and white matter. In conscious, lightly restrained rats, the administration of apomorphine (1 mg . kg-1) effected significant increased in glucose utilization in 15 regions of the CNS (e.g., subthalamic nucleus, ventral thalamic nucleus, rostral neocortex, substantia nigra, pars reticulata), and significant reductions in glucose utilization in two regions of the CNS (lateral habenular nucleus and anterior cingulate cortex). In rats anesthetized with chloral hydrate, the effects of apomorphine upon local glucose utilization were less widespread and less marked than in conscious animals. In only two of the regions (the globus pallidus and septal nucleus), which displayed increased glucose use following apomorphine in conscious rats, were significant increases in local glucose utilization observed with this agent in chloral hydrate-anesthetized rats. In the pars compacta of the substantia nigra, in which apomorphine increased glucose utilization in conscious animals, significant reductions in glucose utilization were observed following apomorphine in rats anesthetized with chloral hydrate. The profound effects of chloral hydrate anesthesia upon local cerebral glucose use, and the modification by this anesthetic regime of the local metabolic responses to apomorphine, emphasize the difficulties which exists in the extrapolation of data from anesthetized animals to the conditions which prevail in the conscious animal.

Effect of apomorphine on the relationship between local cerebral glucose utilization and local cerebral blood flow (with an appendix on its statistical analysis)

The alterations in local glucose utilization and local blood flow in 36 discrete regions of the central nervous system (CNS) that occur following the intravenous administration of the putative dopaminergic agonist, apomorphine (0.5 mg/kg), have been measured using the quantitative autoradiographic 14C-2-deoxyglucose and 14C-iodoantipyrine techniques. In eight of the regions examined (frontal and sensory-motor cortices, ventral thalamus, caudate nucleus, globus pallidus, substantia nigra, subthalamic nucleus, and posterior cerebellar hemisphere), significant elevations of local cerebral blood flow (CBF) were observed following apomorphine administration. In these eight regions, proportionately similar, significant elevations in local glucose utilization were observed following apomorphine. In two of the regions investigated (anterior cingulate cortex and lateral habenular nucleus), significant reductions in both local blood flow and glucose utilization were observed following apomorphine administration. In the majority of regions examined (26 of the total 36), apomorphine did not alter significantly either blood flow or glucose use. Using a statistical approach, described in detail in an appendix, the relationship between local rates of glucose utilization and local levels of tissue blood flow was analyzed. A relationship between local CBF and local glucose utilization was found following apomorphine, and the nature of this relationship was indistinguishable from that observed in control animals. In no region of the CNS was a significant deviation from the normal CBF–glucose use relationship demonstrated following apomorphine administration. These results point to the greater importance of the effects of apomorphine upon tissue metabolic activity, rather than its direct vascular action, as being the major mechanism underlying the observed alterations in local CBF. The statistical methods provide a rigorous analytical approach to the analysis of alterations in the relationship, both locally and globally, of blood supply to glucose utilization.

Effects of the putative GABAergic agonists, muscimol and THIP, upon local cerebral glucose utilisation

The dose-dependent effects of two putative gamma-aminobutyric acid (GABA)-ergic agonists, muscimol and 4,5,6,7-tetrahydroisoxazolo(4,5-e)-pyridin-3-ol (THIP), upon local cerebral glucose utilisation in 60 discrete regions of the CNS have been examined in conscious rats using the [14C]2-deoxyglucose quantitative autoradiographic technique. The intravenous administration of muscimol (0.15-5 mg/kg) and THIP (1-10 mg/kg) resulted in a heterogeneous pattern of significantly reduced glucose utilisation throughout the CNS. The regional hierarchy of changes in glucose utilisation was similar for both muscimol and THIP in all regions (with the exception of the superior colliculus), with muscimol being approximately six times more potent in all regions investigated. The regions in which glucose utilisation was extremely sensitive to change, displaying reductions of approximately 40% following muscimol (1.5 mg/kg) or THIP (10 mg/kg) administration, included all layers of the neocortex (frontal, sensory motor, posterior parietal, primary auditory and visual cortices), the lateral portion of the caudate nucleus, and some thalamic nuclei (lateral geniculate body, mediodorsal and ventrolateral nuclei). Regions displaying more modest reductions in glucose utilisation, approximately 20% following muscimol (1.5 mg/kg) and THIP (10 mg/kg) administration, included most extrapyramidal regions (substantia nigra, pars compacta and reticulata, globus pallidus, subthalamic nucleus, medial portion of the caudate nucleus), a number of cortical and subcortical limbic areas (cingulate and olfactory cortices, hippocampus, nucleus accumbens, anterior thalamus), and medial raphe nucleus. In contrast, in a large number of regions (including cerebellum and related nuclei such as the inferior olivary, red and vestibular nuclei, white matter, pontine reticular formation, hypothalamus, lateral habenula and amygdala), there were only minimal (approximately 10%) reductions in glucose utilisation following muscimol (1.5 mg/kg) and THIP (10 mg/kg) administration. In no region of the CNS was a significant increase in glucose utilisation observed with any concentration of either muscimol or THIP. The regional distribution of alterations in glucose utilisation following muscimol and THIP administration, which does not correspond to the known topography of GABAergic neurones and receptors, provides a comprehensive description of the functional alterations, as reflected in rates of glucose utilisation, that occur in conscious rats after systemic administration of these two putative GABAergic agonists.

Distribution of effects of haloperidol on energy metabolism in the rat brain

The effects of the putative dopaminergic antagonist, haloperidol (0.01-10 mg/kg, i.v.), on cerebral glucose utilization in 43 anatomically discrete regions of the rat brain have been examined by the quantitative autoradiographic 2-deoxyglucose technique. Dose-dependent reductions in glucose utilization were observed in 10 regions of the CNS (e.g. hippocampus, ventral thalamus and almost the entire neocortex, with the notable exception of anterior cingulate cortex). Two regions of the CNS (nucleus accumbens and pars compacta of the substantia nigra) displayed dose-related increases in glucose utilization following haloperidol administration. In addition to these specific alterations, the largest doses of haloperidol produced widespread, moderate (about 25%) reductions in glucose utilization throughout the CNS. The prior administration of haloperidol (0.1 mg/kg) prevented the effects on glucose utilization of the administration of apomorphine (1.5 mg/kg) in all regions of the CNS examined. The distribution of alterations in glucose utilization following haloperidol administration are considered in relation to the overall functional consequences of dopaminergic receptor blockade.

Superior colliculus activation by retinal nicotinic ganglion cells: a 2-deoxyglucose study

Systemic injection of the acetylcholinesterase inhibitor, di-isopropylfluorophosphate, in rats causes a marked increase in glucose use in the superficial layers of the superior colliculus. This activation of the superior colliculus is largely a retinal effect. Furthermore, since this response can be blocked by intraocular as well as systemic injections of mecamylamine, it is postulated that retinal nicotinic receptors are involved.

The distribution of alterations in energy metabolism in the rat brain produced by apomorphine

The effects of the putative dopaminergic agonist, apomorphine (0.15-5 mg/kg, i.v.), on glucose utilization in 43 anatomically discrete regions of the rat brain have been examined by the quantitative autoradiographic 2-deoxyglucose technique. Apomorphine failed to alter the rates of glucose utilization in 25 of these regions (for example, primary auditory areas, regions of white matter, hippocampal areas, nucleus accumbens and caudal regions of the neocortex). Dose-dependent alterations in glucose utilization were observed following apomorphine administration in a number of regions known to contain dopaminergic receptors (viz: caudate nucleus, substantia nigra, amygdala, subthalamic nucleus and anterior cingulate cortex). Moreover, dose-dependent alterations in glucose utilization were produced by apomorphine in a number of regions thought to contain few specific dopaminergic receptors (e.g., cerebellar hemisphere and vermis, lamina VI of rostral neocortical areas, and ventral nucleus of the thalamus). The distribution of alterations in glucose utilization following apomorphine administration are considered to reflect the functional involvement of the region in the overall response to apomorphine, and not simply the topography of dopaminergic receptor mechanisms.

Suppression of apomorphine-induced oral stereotypy in rats by microinjection of muscimol into midbrain

Rats with large lesions of the superior colliculus do not display the oral stereotypy normally induced by high systemic doses of dopamine-agonists. It has been suggested that collicular lesions have such an effect because they destroy the GABAergic nigrotectal pathway. This suggestion was investigated by observing the effects of bilateral microinjections of the GABA-agonist muscimol into midbrain sites in rats given 8 mg/kg subcutaneous apomorphine. A low dose of muscimol (25 ng in 0.5 ul saline/side) injected into regions of the superior colliculus with nigrotectal innervation almost abolished apomorphine-induced licking and gnawing. Control microinjections of saline into the superior colliculus, or of muscimol into overlying cerebral cortex, were ineffective. This result is consistent with the GABAergic nigrotectal projection being important for the expression of dopamine-related oral stereotypy. It was also found, however, that 25 ng of muscimol suppressed oral stereotypy when microinjected into the mesencephalic reticular formation underlying the superior colliculus. The anatomical basis of this latter effect is uncertain.

Kainic acid alters cholinergic responses in the rat retina: a 2-deoxyglucose study

Intraocular injections of the neuroexcitatory toxin, kainic acid, did not alter the output of the retinal ganglion cells, as determined by the rate of glucose use in the stratum griseum superficialis of the superior colliculus. However, significant differences were observed in cholinergic interactions of the ganglion cells after kainic acid treatment. Intraocular injection of kainic acid prevented the increase in the stratum griseum superficialis activity typically produced by systemic injection of the acetylcholinesterase inhibitor diisopropylfluorophosphate (DFP). In addition, the retinal ganglion cells were strikingly sensitive to intraocular injections of acetylcholine 1 week after exposure to kainic acid, as reflected in the marked increased glucose utilization in the stratum griseum superficialis. This responsiveness to acetylcholine may be entirely due to the 80% decrease in acetylcholinesterase in the retina observed 1 week after kainic acid exposure or in part to a supersensitivity of the ganglion cells following the period of acetylcholine depletion.

The relationship of glucose utilization and morphological change in the visual system in hexacarbon neuropathy

A reduction in local glucose utilization occurs in the superficial layer of the superior colliculus of rats following exposure to 0.5% 2,5-hexanedione in drinking water for 3 weeks. Axonal pathology, with increased neurofilaments and swelling, is seen at about 5 weeks in the distal portions of the optic pathways to the superior colliculus. These lesions in the optic system occur earlier and are more marked than those in the peripheral nerves and dorsal columns. The results suggest that the functional abnormality, as implied by the decreased glucose utilization, precedes the morphological changes in hexacarbon neuropathy.

Heterogeneous depression of glucose utilization in the caudate nucleus by GABA agonists

Local cerebral glucose utilization in the rat caudate nucleus has been examined following the intravenous administration of the GABA agonists muscimol and THIP. Although glucose utilization was reduced throughout the striatum following GABA agonists, the magnitude of the reductions in the lateral portion of the nucleus were markedly greater than in other portions of the caudate nucleus. This medial--lateral heterogeneity in response to GABA agonists contrasts with the known rostral--caudal heterogeneity in the distribution of biochemical markers for GABA.

Gnawing and changes in reactivity produced by microinjections of picrotoxin into the superior colliculus of rats

Lesions of the superior colliculus in rats attenute the oral stereotypy produced by systemic administration of dopamine agonists. Current evidence suggest that such drugs affect the superior colliculus by reducing transmission in the inhibitory GABAergic nigrotectal pathway. To investigate whether the superior colliculus plays a direct role in producing stereotyped oral movements, the present experiment therefore examined the effects of collicular microinjections of the GABA antagonist picrotoxin on the behaviour of rats observed in an open-field. Gnawing was observed after injections of picrotoxin (25 ng) into sites in the intermediate and deep layers of the superior colliculus, consistent with the superior colliculus playing a direct part in producing the stereotyped gnawing seen after systemic administration of dopamine agonists. However, gnawing was only observed after a period in which the animal showed strong avoidance reactions to stimuli that normally evoked orienting or little reaction. This change in reactivity was observed after injections of picrotoxin into all parts of the colliculus, but the most sensitive (responding to doses as low as 12.5 ng) were mainly in the superficial and intermediate layers. It appears that there may be more than one GABAergic system within the superior colliculus.

Collicular picrotoxin alleviates akinesia but not sensory neglect in rats with bilateral 6-hydroxydopamine lesions of ventral midbrain

Anatomical and biochemical investigations have suggested that GABA transmission in the superior colliculus consequent upon activity of the nigrotectal pathway is increased following 6-hydroxydopamine (6-OHDA) lesions of the ascending dopamine systems. Moreover, it has been proposed that this increase in inhibitory activity within the colliculus may be responsible for the sensory neglect commonly observed after dopamine denervation. The present experiment sought to test this proposal by examining the effects of injections of the GABA antagonist picrotoxin into the superior colliculus of 6-OHDA lesioned rats, in the hope that the neglect caused by the 6-OHDA would be reversed. However, in 33 of 36 cases studied intracollicular microinjections of picrotoxin produced no detectable improvement in orientation to sensory stimuli, although a wide range of other behavioural effects was observed. These included stereotyped exploratory movements (e.g. head waving, walking, sniffing and rearing) similar to those produced in the 6-OHDA treated rats by systemic injection of apomorphine (0.1 mg/kg). These data indicate that 6-OHDA lesions of substantia nigra and ventral midbrain areas do not produce sensory neglect simply by increasing GABA transmission within the nigrotectal pathway. Instead, such an increase in nigrotectal activity may impair production of particular kinds of movement, possibly related to exploratory behaviour.